1. Field of the Invention
The invention relates in general to a method of reading a dual-bit memory cell, and more particularly to a method of reading a dual-bit nitride trapping layer memory cell.
2. Description of the Related Art
Please refer to FIG. 1. FIG. 1 is a cross-sectional view of a nitride trapping layer memory cell. In order to achieve a memory device with high density, a dual-bit nitride trapping layer memory cell 10 is developed to replace a conventional single-bit memory cell using a floating gate. As shown in FIG. 1, the nitride real-only memory cell 10 includes a bulk 15 implanted with a source 12 and a drain 14. An oxide-nitride-oxide (ONO) structure 16 and a gate 13 are formed over the bulk 15. The ONO structure 16 includes a top oxide layer 16a, a nitride layer 16b and a bottom oxide layer 16c stacked together.
The nitride layer 16b (such as silicon nitride) of the ONO structure 16 functions as a non-conductive charge-trapping layer. Accordingly, when a programming operation is performed, electrons are injected and captured on two sides (adjacent to the source 12 and the drain 14 respectively) of the nitride layer 16b. As a result, a first bit storage node B1 and a second bit storage node B2 are formed on the two sides of the nitride layer 16b respectively, as shown in the area circled by the dotted line in FIG. 1. A single-bit logic 1 or 0 is stored respectively according to whether electrons are captured. Through the first bit storage node B1 and the second bit storage node B2, the nitride trapping layer memory cell 10 can store two bits at a time. Preferably, the bit states of the first bit storage node and the second bit storage node are independently accessed by comparing an output current value and a predetermined reference value of the nitride trapping layer memory cell 10 through reverse read. However, a second-bit effect affects the output current value of the nitride trapping layer memory cell 10 and limits the setting range of the predetermined reference value significantly. As a result, the sensing window is decreased.